This invention is related to an electric power steering system which produces a steering assisting force by an electric motor.
An electric power steering system (EPS) for automobiles employs a reduction mechanism. In a column type EPS, for example, the revolution of the output shaft of the electric motor is reduced through a worm shaft and a worm wheel, whereby the output of the electric motor is amplified and is transmitted to a steering mechanism to assist the torque in the steering operation.
A suitable degree of backlash is necessary for the meshing between the worm shaft and the worm wheel. When, for example, traveling on a rough road, however, there may generate teeth-hitting sound (rattling sound) due to the backlash reversely input from the road surface.
To suppress the generation of teeth-hitting sound, it becomes necessary to strictly adjust the amount of backlash that occurs between the worm shaft and the worm wheel within a range of machining precision of the parts. In assembling the worm shaft and the worm wheel, so far, it is a practice to select the individual parts depending upon the degree of dispersion in the dimensional precision and assemble those which assume suitable degrees of precision when combined together (so-called matching assembly) requiring, however, very laborious work and boosting up the cost of production.
Further, even if the backlash may, at first, have been set in a suitable range owing to the matching assembly, the backlash increases due to wear of the teeth after aging producing abnormal sound.
Therefore, there has been proposed an art for decreasing the backlash by permitting the bearing that supports an end of the worm shaft to advance or retreat to or from the worm wheel in the radial direction, urging the end of the worm shaft toward the worm wheel in the radial direction by using a curved leaf spring that is covering the periphery of the outer wheel of the bearing, and automatically adjusting the distance between the center of rotation of the worm shaft and the center of rotation of the worm wheel (so-called inter-core distance) (see, for example, patent document 1).
There has further been proposed an art for adjusting the backlash by converting part of the urging force of the compression coil spring that urges the worm shaft in the axial direction into an urging force for urging the worm shaft toward the worm wheel in the radial direction (see, for example, patent document 2).    Patent document 1: JP-A-2004-203154    Patent document 2: JP-A-2002-145082
According to the former publication, however, the curved leaf spring often rattles due to the clearance in a direction other than the urging direction. In this case, the rotational torque of the worm shaft fluctuates and the durability of the curved leaf spring decreases. There further remain problems in that sliding sound or hitting sound is generated between the worm shaft and the worm wheel, or hitting noise is generated between the curved leaf spring and the housing.
According to the latter publication, on the other hand, the urging force is obtained in two directions by using a single compression coil spring. Therefore, the structure of the cam mechanism tends to become complex and the operation tends to lose stability.